5 Steps to a 5 AP Chemistry

concentration of CO is doubled. This direct

change means the reaction is first order with

respect to CO. Comparing Experiments 1 and 3,

the rate doubles when the concentration of Cl 2 is

doubled. Again, this direct change means the

reaction is first order. This gives: Rate =

k[CO]^1 [Cl 2 ]^1 =k[CO][Cl 2 ].

10. C—The compound appears in the rate law, and
    so a change in its concentration will change the
    rate. The reaction is first order in (CH 3 ) 3 CBr, so
    the rate will change directly with the change in
    concentration of this reactant. There is no equi-
    librium arrow, so the reaction is not in equilib-
    rium. If the reaction were in equilibrium were in
    equilibrium, D would also be true.
    11. B—All substances involved, directly or indirectly,
    in the rate-determining step will change the rate
    when their concentrations are changed. The ion
    is required in the balanced chemical equation, so
    it cannot be a spectator ion, and it must appear
    in the mechanism. Catalysts will change the rate
    of a reaction. Since H+does not affect the rate,
    the reaction is zero order with respect to this ion.
    12. E—The rate law depends on the slow step of the
    mechanism. The reactants in the slow step are Cl
    and CHCl 3 (one of each). The rate law is first
    order with respect to each of these. The Cl is half
    of the original reactant molecule Cl 2. This
    replaces the [Cl] in the rate law with [Cl 2 ]1/2. Do
    not make the mistake of using the overall reac-
    tion to predict the rate law.

208  Step 4. Review the Knowledge You Need to Score High

 Free-Response Questions

You have 15 minutes for this question. You may use a calculator and the tables at the back

of the book.

2ClO 2 (aq) +2 OH−(aq) →ClO− 3 (aq) +ClO 2 −(aq) +H 2 O(l)

A series of experiments were conducted to study the above reaction. The initial concen-

trations and rates are reported in the table below.

INITIAL CONCENTRATION (mol/L) INITIAL RATE OF

FORMATION OF ClO 3 −

EXPERIMENT [OH−] [ClO 2 ] (mol/L min)

1 0.030 0.020 0.166

2 0.060 0.020 0.331

3 0.030 0.040 0.661

a. i. Determine the order of the reaction with respect to each reactant. Make sure you

explain your reasoning.

ii. Give the rate law for the reaction.

b. Determine the value of the rate constant, making sure the units are included.

c. Calculate the initial rate of disappearance of ClO 2 in experiment 1.

d. The following has been proposed as a mechanism for this reaction.

Step 1: ClO 2 +ClO 2 →Cl 2 O 4

Step 2: Cl 2 O 4 +OH−→ClO 3 −+HClO 2

Step 3: HClO 2 +OH−→ClO 2 −+H 2 O

Which step is the rate-determining step? Show that this mechanism is consistent with

both the rate law for the reaction and with the overall stoichiometry.